EVO: Engine Mechanicals

Cylinder Pressure - Compression

Cylinder Pressure Test

It is important to make a check of your cylinder pressure (aka engine compression) as soon as you obtain your bike and, at least, on an annual basis thereafter. This is a great reference value to determine if the pistons, rings, cams, lifters, pushrods and valves are all functioning correctly together. Over time, these parts wear and your cylinder compression will reflect when these items should be checked, adjusted or replaced.

New compression readings should be taken whenever any type of headwork, valve reseating, new head gaskets, etc., is performed in order to evaluate the operational functioning and to have a new reference point for the future.

To check your cylinder pressure, you need a compression tester/gauge. For less than $25, you can buy (online) a tester with a gauge, hose and adapters (slightly more at your local auto supply or department store).

Run the engine to get it up to operating temperature (maybe a 10-15 minute ride)

Let the engine cool down just enough so you can work with it

Disconnect and remove both spark plugs

Screw the compression tester into the spark plug hole for the front cylinder

Lock the throttle open

Critical - Remove the air cleaner & hold the CV Carb slide up (other carbs, make the choke fully open)

Crank the engine for several rotations until maximum pressure on gauge

Record the final reading

Repeat this procedure for the rear cylinder

Results The difference between the two readings should not be more than 10%. A low reading on either cylinder indicates a possible valve or piston ring issue. To determine which, pour about 1/2 teaspoon of oil into that cylinder through the spark plug hole. Then repeat the compression test. If the reading returns to normal the rings may be defective. If the compression does not increase to within 10% then the valves of that cylinder may require service.

Cylinder Leakdown Test

If you find you have low compression in a cylinder, you will want to do a Leakdown Test. This test will help determine the location of the leaking air, such as worn exhaust valves, worn intake valves, leaky head gaskets, or worn rings.

The test should be donw when the engine is at or close to normal operating temperature, such as right after doing the compression test.

To perform the test, you will need an air compressor, a Leakdown Tester (around $50 online) and something to listen for air leaks, like a mechanics stethoscope (with the probe removed) or a length of oil line hose. (A small air compressor can be had for under $100, but if you purchase one that can produce 5cfm or more at 90psi, you can use it for some air tools as well.)

Listen at the timing plug hole. Excessive hissing here indicates the rings are not sealing

Listen around the cylinder/head gasket. Excessive hissing here indicates the gasket is not sealing

Greater leakage of air results in the sound being of a lower pitch.

There are many variables here [engine temperature, precise piston position, whatever]. The test results will not be consistent from one test to the next, so multiple tries should be recorded and a consensus value established.

When cylinder requires boring beyond 0.040 in 883cc or 1100cc or beyond 0.30 in 1200 cylinders, the oversize limit has been exceeded and the cylinder must be replaced.

Boring and honing must be done with gaskets and torque plate in place. Bore the cylinder to 0.003 in under the desired finished size. Hone the cylinder to match the piston clearance using a 280 grit rigid hone followed by a 240 grit ball hone. Hone from the bottom end (crankcase side) of cylinders working for 60 degree cross hatch pattern.